1. Field of the Invention
The present invention relates generally to a mobile radio communication system, and more particularly to an apparatus and method of linearizing the characteristic of a power amplifier in the mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system.
2. Description of the Related Art
A high-power amplifier which is used for transmitting analog data or digital data in mobile radio communication systems requires a high spectrum efficiency as well as a high power efficiency in order to construct a low-power consuming system in a limited frequency band. In order to meet such general requirements in the system, baseband data modulation methods such as QPSK and QAM having a high spectrum efficiency have been used. Also, a high-efficiency power amplifier such as a class C amplifier has been used to improve the power efficiency of a transmitter in the system. Such a high-efficiency power amplifier generally has strong non-linear characteristics, consequently producing a distortion phenomenon, such as a sidelobe reproduction, in its output spectrum. This phenomenon is especially prevalent in the case where a modulated signal such as a QPSK or QAM signal, which does not have a constant envelope characteristic, passes through the power amplifier with the non-linear characteristic.
Various methods have been proposed for preventing the distortion of the output spectrum of the power amplifier resulting from the non-linear characteristics of the power amplifier. One among them is a method of compensating for the non-linear characteristic of the high-power amplifier by adaptively tracking the non-linear characteristic of the power amplifier, predistorting the baseband data in a manner opposite to the distortion caused by the non-linear characteristic of the power amplifier.
FIG. 1 is a block diagram illustrating the construction of a conventional power amplifier employing the above-described adaptive predistortion method.
Referring to FIG. 1, K-bit data encoded by an encoder (not illustrated) is inputted to a shift register 10 and a modulation select read only memory (ROM) 52. The shift register 10 has the length of an L-symbol span, its output has a size of LK bits. At this time, if there is no linear distortion caused by the filtering operation of a filter (not illustrated) existing in the system and the length of one symbol is enough for the length L of the shift register 10, while if a linear distortion due to the system filtering exists, the length of the shift register 10 should be longer than one symbol.
The LK-bit output of the shift register 10 is inputted to a predistort RAM 12. This predistort RAM 12 is stored with predistortion data mapped for data outputted from the shift register 10. The predistortion data is updated in accordance with input error data. Upon receiving data from the shift register 10, the predistort RAM 12 outputs predistortion data corresponding to the received data. That is, the predistort RAM 12 predistorts data outputted from the shift register 10 using error data having a phase opposite to a distortion of the transmission signal so that a radio frequency (RF) transmission section detects the distortion of the transmission signal and compensates for the detected distortion. The output of the predistort RAM 12 is converted to an analog signal for transmission by an I-channel digital-to-analog (D/A) converter 14 and a Q-channel digital-to-analog (D/A) converter 16, respectively. The analog signals converted by the respective D/A converters 14 and 16 are low-pass-filtered through low-pass filters (LPFs) 18 and 20, and then inputted to a quadrature modulator 22. The analog signals inputted to the quadrature modulator 22 are mixed with an output of a first oscillator 32, and then modulated to an intermediate frequency signal in the quadrature modulator 22. The intermediate frequency signal modulated by the quadrature modulator 22 is determined by the first intermediate frequency (IF) oscillator 32, and is mixed with an output of a second oscillator 28 by a mixer 24 to be converted to a final radio frequency (RF) transmission frequency. The RF frequency signal outputted from the mixer 24 is finally amplified by a power amplifier 26, and then transmitted through an antenna.
A portion of the output of the power amplifier 26 is fed back to a mixer 30 by a signal coupler 54, and the fed-back signal is mixed with the output of the second oscillator 28 by the mixer 30 to be converted to a first IF signal. The converted IF signal is then converted to baseband data by a quadrature demodulator 34 using a local oscillation signal outputted from an IF oscillator 32.
The baseband signal converted by the quadrature demodulator 34 is compared with each output signal of D/A converters 48 and 50, which is used as a reference signal for generating an error signal, by analog adders 40 and 42, respectively. Here, output signals of the modulation select ROM 52 are inputted to the D/A converters 48 and 50, and used as reference signals for comparing with the signals fed back to update the value of the predistort RAM 12. The reference signals added in the analog adders 40 and 42 and the fed-back signals are respectively converted to digital signals, and then added to the digital signals of the predistort RAM 12 by digital adders 36 and 46 to update the value of the predistort RAM 12. Error data outputted from the digital adders 36 and 46 are inputted to the predistort RAM 12 via a data bus, and then stored in addresses determined by the shift register 10 to complete the predistortion process with respect to the baseband data.
However, there are some disadvantages according to the conventional predistortion method shown in FIG. 1. First, the shift register 10 for generating addresses and the modulation select ROM 52 for obtaining the reference signal required for generating the error signal must be employed. Also, the high-accuracy adders 40 and 42 for obtaining the error signal must be employed to update the value of the predistort RAM 12. Constructing such high-accuracy analog adders is difficult, and is highly dependant on accuracy. In addition, it is generally known that the performance obtained by the predistortion method is lower than that obtained by a feedforward method.